This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-353113, filed on Nov. 20, 2000; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser, and is suitably applicable particularly to an InGaAlP quantum well structure surface emitting semiconductor laser.
2. Description of the Related Art
Recently, a semiconductor laser emitting in the range of wavelengths from 600 to 700 nm has been put into practical use in a field such as DVD (digital versatile disk) or the like.
Meanwhile, a surface emitting semiconductor laser, if being put into practical use in this wavelength region, will be used as a light source for a high speed plastic fiber link. As a surface emitting semiconductor laser realizing such wavelength region, there is an InGaAlP surface emitting semiconductor laser. In order to lower a threshold current of the InGaAlP surface emitting semiconductor laser, there is a surface emitting semiconductor laser in which active layer a quantum well structure is adopted.
FIG. 10A is a perspective view showing a configuration of an existing InGaAlP quantum well surface emitting semiconductor laser. In FIG. 10A, on an n-GaAs substrate 1, there are stacked in turn a DBR (Distributed Bragg Reflector) multi-layer film 2, an n-InGaAlP clad layer 3, an MQW (Multiple Quantum Well) active layer 4, a p-InGaAlP clad layer 5, a DBR multi-layer film 6, and a p-GaAs cap layer 7. On a back surface of the n-GaAs substrate 1, there is formed an n-side electrode 8, on the p-GaAs cap layer 7 there being formed a p-side electrode 9. In the p-GaAs cap layer 7 and the p-side electrode 9, there is formed a disk like opening to form a light exit window 10 for taking out emitted light.
The active layer 4 is formed of In0.5Ga0.5P/In0.5(Ga0.5Al0.5)0.5P film; the clad layer 3 being formed of an n-In0.5(Ga0.3Al0.7)0.5P film; the clad layer 5 being formed of a p-In0.5(Ga0.3Al0.7)0.5P film; the DBR multi-layer film 2 being formed of an n-Ga0.5Al0.5As/Ga0.05Al0.95As film; the DBR multi-layer film 6 being formed of a p-Ga0.5Al0.5As/Ga0.05Al0.95As film.
Furthermore, the active layer 4 and the clad layers 3 and 5 form a resonator of the surface emitting semiconductor laser, and at the active layer 4 in the center a film thickness is designed to be an antinode of a standing wave of one wavelength.
FIG. 10B is an enlargement of a portion of the active layer and the clad layers in FIG. 10A. In FIG. 10B, the MQW active layer 4, which is formed by repeating to alternately stack an In0.5(Ga0.5Al0.5)0.5P film 4a and an In0.5Ga0.5P film 4b, is sandwiched by the clad layers 3 and 5 to form a double heterostructure junction.
FIG. 10C is an energy band diagram of a portion of the active layer 4 and the clad layers 3 and 5. In FIG. 10C, the In0.5Ga0.5P films 4b being smaller in their band gaps in comparison with the In0.5(Ga0.5Al0.5)0.5P films 4a, there are formed quantum wells QW at the portion of the In0.5Ga0.5P films 4b. In the quantum wells QW, energy levels are quantized, and thereby energy levels of electrons injected into the active layer 4 may be localized. As a result, the laser may be efficiently oscillated, the threshold current being lowered.
In addition, the clad layers 3 and 5, which are formed of the n-In0.5(Ga0.3Al0.7)0.5P film, are larger in their band gaps than those of the In0.5Ga0.5P/In0.5(Ga0.5Al0.5)0.5P films. As a result, electrons and holes injected through the clad layers 3 and 5 may be confined inside the active layer 4, the laser being efficiently oscillated.
In the existing MQW structure surface emitting semiconductor laser, in order to enhance a gain, it is general to set a well width Hb at 7 nm or more and the number of wells Wn at five or more. However, in the active layer 4 adopting the MQW structure, when the well width is 7 nm or more and the number of the wells is five or more, heat generation from the active layer 4 becomes larger, resulting in deterioration of high temperature properties.
The object of the present invention is to provide a surface emitting semiconductor laser capable of improving the high temperature properties.
A surface emitting semiconductor laser according to an embodiment of the present invention includes an active layer having an InGaAlP quantum well structure of which well width is from 4 nm to 6 nm and of which number of wells is one or two, InGaAlP clad layers formed above and below the active layer, and light reflecting layers formed, in a stacking direction of the active layer, further above and below the clad layers through the respective clad layers.